Over the past many decades, commercial interest in alkylated diamines has been frustrated by the difficulty of separating a desired or `target` diamine in a mixture (first) of alkylated diamines all of which have molecular weights in so narrow a range as to make separation of individual diamines in such a first mixture impractical. Frustration in making a desired separation is evidenced by the high cost of obtaining suitably pure precursors for the preparation of a wide spectrum of amine-based compounds, for example, piperidine, piperazine, and piperazinone-based hindered amine light stabilizers.
Only the target diamine or bis-compound thereof (referred to herein as a "bis-target diamine") is desired for the preparation of a polysubstituted 1,4-diazacycloheptan-2-one disclosed in Lai U.S. Pat. Nos. 4,167,512; 4,297,497 and 4,466,915, the disclosures of which, particularly those relating to the ketoform reaction, are incorporated by reference thereto as if fully set forth herein. In a manner analogous to that illustrated in Exs 1-3 of the '915 patent, it is most desirable to start with the target diamine or bis-target diamine in essentially pure form. By "essentially pure" diamine we refer to a purity of at least 90% by weight. For example, the target diamine N.sup.1 -butyl-4-methyl-2,4-pentanediamine, is reacted with a ketone and chloroform in the presence of an alkali metal hydroxide, with or without a phase transfer catalyst, to form N.sup.1 -(butyl)-3,3,5,5,7-pentamethyl-1,4-diazepin-2-one having a seven-membered ring. In such a ring, each of the N.sup.4 -adjacent 3- and 5-carbon atoms is disubstituted so that the N.sup.4 atom is said to be "fully" hindered. However, making the appropriately substituted target diamine or bis-target diamine in essentially pure form is an impractical and difficult task.
The N.sup.4 atom in the 1,4-diazacycloheptan-2-ones is referred to as being "fully hindered" when it is flanked by adjacent disubstituted C atoms: the N.sup.4 atom is referred to as being "partially" hindered when it is flanked by one adjacent disubstituted C atom and one monosubstituted C atom.
The task is impractical because, despite the alkylation of a 1-disubstituted-1,3-propanediamine (referred to as the "starting" diamine) being relatively straightforward, the reaction between alkyl halides and diprimary amines does not produce only the "expected" alkylated diamine. The reaction does not stop after alkylation of the less hindered primary amine group in the starting diamine, but also alkylates the hindered primary amine group. Thereafter the secondary amine groups of the alkylated starting diamine are further alkylated forming oligomers of the starting diamine. By "expected" diamine we refer to one which would be expected to form in a textbook application of an alkylation reaction.
One particular target diamine of interest herein is mainly used as a precursor for 1,4-diazacycloheptan-2-one-based light stabilizers disclosed in references hereabove. The target diamine is represented by the structure ##STR1## wherein, R.sup.1, R.sup.2, and R.sup.3 independently represent C.sub.1 -C.sub.24 alkyl (having from 1 to 24 carbon atoms);
R.sup.3, but only one of R.sup.1 and R.sup.2, represent C.sub.7 -C.sub.20 aralkyl;
R.sup.4 represents C.sub.1 -C.sub.20 alkyl, C.sub.5 -C.sub.24 cycloalkyl or alkyl-substituted cycloalkyl, the ring being C.sub.5 -C.sub.8 ; C.sub.7 -C.sub.20 aralkyl, and R.sup.1 R.sup.2 C.dbd.CR.sup.1 -CH.sub.2 --; and,
R.sup.1 and R.sup.2 together when cyclized represent C.sub.5 -C.sub.7 cycloalkyl.
Also of particular interest is a bis-target diamine used as a precursor for a bis-1,4-diazacycloheptan-2-one light stabilizer in which polysubstituted 1,4-diazacycloheptan-2-one moieties are connected by a divalent radical. The bis-target diamine is represented by the structure ##STR2## wherein, R.sup.5 represents C.sub.2 -C.sub.24 alkylene, R.sup.5 -substituted C.sub.9 -C.sub.23 cycloalkyl the cycloalkyl ring being C.sub.5 -C.sub.8 ; and,
R.sup.8 --Ph--R.sup.8 wherein Ph represents phenyl and R.sup.8 represents C.sub.2 -C.sub.12 alkylene.
In one embodiment, this process capitalizes on the ability to reductively alkylate only those diamines which one wishes to exclude from the feed to a subsequent ketoform reaction, so that the target diamine may be conveniently separated from the reductively alkylated mixture.
In another embodiment, this process side-steps the problem of separating the individual components of the alkylated first mixture by using the entire mixture of alkylated diamines in a ketoform reaction which results in conversion of essentially all the target diamine to desired 1,4-diazacycloheptan-2-one product. The 1,4-diazacycloheptan-2-one product is unexpectedly easy to isolate from a reaction mass which is a mixture (second) containing unreacted ketone and diamine reactants, alkali metal hydroxide, phase transfer catalyst, alkali metal salt and the 1,4-diazacycloheptan-2-one product among other unwanted cyclic and acyclic diamines.
Diprimary amines of particular interest having a hindered N atom, are represented by the structure ##STR3## These are commercially available 1-disubstituted-1,3-propanediamines used as starting materials for a conventional alkylation reaction which produces the first mixture of alkylated diamines.
With particular respect to the diamine (I), though the disubstituted N.sup.2 -adjacent C atom provides a measure of steric hindrance to the N.sup.2 -amino group, it is nevertheless alkylated to a surprisingly large extent, in preference to the N.sup.1 -amino group even when only one mol of alkylating agent is used for two mols of starting diamine. If only one or the other of the primary amino groups of the starting diamine is alkylated with a monohalide, the alkylated diamine contains one secondary amino group. In addition, both primary amino groups are found to be alkylated, resulting in some of the alkylated product having terminal secondary amino groups. This clearly indicates that even when the starting diamine and alkylating agent are present in a molar ratio of 2:1 the N.sup.1 and N.sup.2 atoms are both alkylated to a surprisingly large extent.
If the halide is a dihalide, not only bis-compounds are formed having both primary and secondary groups, or, only secondary amino groups, but each of these may be further alkylated (as shown herebelow) producing still more unwanted byproduct diamines including oligomers having from 3 to about 6 diamine repeating units.
Therefore when both amine groups are primary, or when both primary and secondary amine groups are present in the amine to be alkylated, a wide assortment of alkylated products is formed even under the most controlled conditions (see "Advanced Organic Chemistry Reactions, Mechanisms and Structures" by J. March, 3d edition, btm of pg. 365 John Wiley & Sons 1984.) For this reason, generally, alkylation with an alkyl halide is used where a tertiary amine is desired and one expects to effect complete alkylation of all amine groups. Even carefully controlled conditions generally give a mixture of alkylated products and is not favored even on a laboratory scale.
To be sure, one might expect a much larger amount of the N.sup.1 -amino group to be alkylated in preference to the N.sup.2 -amino group, if one used substantially less than a molar equivalent of the alkylating agent, with the expectation that essentially all the unhindered N.sup.1 -amino group will be alkylated first. It is not. For example, only 0.5 mol of monohalide R.sup.4 X or XR.sup.5 X (defined herebelow) per mole of starting diamine will result in so large an assortment of alkylated products as to make using the first mixture impractical. Yet, by using a larger molar excess of starting diamine than 3:1, the process of this invention effectively produces a major molar proportion of the alkylated diamine in which the N.sup.2 -amino group is not alkylated. The alkylated diamines in the first mixture then yield an essentially pure product of 1,4-diazacycloheptan-2-ones which are recovered from a reaction mixture ("second" mixture) resulting from subjecting the first mixture to a ketoform reaction, explained in detail in the aforesaid Lai patents.
By an "essentially pure" product we refer to one in which the polysubstituted 1,4-diazacycloheptan-2-ones containing either a fully or partially hindered N.sup.4 atom, constitute at least 90% by weight. In the product, the 1,4-diazacycloheptan-2-one with the fully hindered N.sup.4 atom is preferentially formed, that is, it is present in a major molar amount relative to the amount of that with a partially hindered N.sup.4 atom due to an idiosyncracy of the ketoform reaction. This idiosyncracy is at least in part attributable to the combination of the N.sup.2 -adjacent disubstituted C atom and the required presence of a substituent on the N.sup.1 -adjacent C atom.
In some instances, despite using more than a threefold, and preferably more than a four-fold excess, the first mixture still contains too many alkylated byproducts to provide a practical feedstream for the ketoform reaction. It is therefore necessary to concentrate the target diamine and remove it from the first mixture. It was found that, if the first mixture is reductively alkylated with a relatively high molecular weight halide, the reductively alkylated products are so physically different from the target diamine that the latter is easily separable. This is because, quite unexpectedly, the target diamine is not reductively alkylated, thus making it easy to distill, or otherwise separate it from the resulting mixture.
The reductive alkylation of acyclic diamines is well known and described with numerous examples, in the chapter entitled "Preparation of Amines by Reductive Alkylation" by W. S. Emerson in Organic Reactions, Vol 4, John Wiley & Sons, New York, N.Y. Examples are given for preparation (A) of tertiary amines from (i) secondary aliphatic amines and ketones, (ii) aryl alkyl amines and aliphatic aldehydes, (iii) aryl alkyl amines and ketones; etc., and, (B) of secondary amines by (i) reduction of Schiff's bases derived from aliphatic amines, and from aromatic amines, and (ii) reduction of primary aromatic amines, nitro or nitroso compounds and ketones, etc. In reductive alkylations with an aldehyde there is a wide scatter of side reaction because of the higher reactivity of an aldehyde than a ketone. There is no teaching that reductive alkylation of a substituted 1,3-propanediamine with a ketone will result in alkylation only at secondary amino groups, and at a primary amino group having a monosubstituted adjacent C atom. With particular respect to a reductive alkylation of diamines, triamines, tetramines, or polyamines generally, nowhere is there any suggestion that such a reaction may be used as a `dynamic sieve` which allows only the desired target diamine to be recovered unaffected.